The 50mm f/1.2L is something of a special case. That lens suffers from a particularly bad focus shift (many lenses have some focus shift, especially, fast primes, but usually not enough to notice). In the case of the 50L, the focus shift is an optical design tradeoff for the undercorrected spherical aberration that gives the lens its legendary creamy bokeh.

Focus shift is when the focal plane of the lens changes when you change the aperture of the lens. All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken. In the case of the 50mm f/1.2L, if you select an aperture narrower than f/1.2, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane. At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF. At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).

So, what can you do? Here are some options:

Stop down to f/4 or narrower. But, I'm going to assume that shooting at f/8 is not the ideal solution...

Shoot at f/1.2. There's no shift if you don't stop down.

Use Live View. Granted, that's not a good option with typical 50/1.2 subjects.

More complex AFMA. You could perform an AFMA at f/2, and you'd get different results than wide open - that adjustment would compensate for the focus shift at f/2, but not be applicable at f/1.6 or f/2.8, for example. So, you'd need to have a list of AFMA values, and change the setting to match the aperture you want to shoot at. Might work if you can pick an aperture for a shoot and stay there. (Side note here: the 1D X can store two AFMA settings for a zoom lens, one for the wide end and one for the long end; in theory, Canon could allow multiple, aperture-dependent AFMA settings for the 50L, which would certainly help with this issue.)

Intentionally front focus. Use an AF point over a feature that's a little bit in front of what you really want to focus on.

Tweak on the fly. The 50L has full-time manual focus, so you can use AF get you close, then turn the MF ring slightly to bring the focal plane forward a little. You'd likely want to be using back-button AF for that, and it would take a fair bit of practice to get it working reliably.

The 50L can deliver amazing shots, but due to the focus shift issue it takes some work and practice to get the most from the lens.

The 50L, I consider an artist lens. Not in the sense you need to be an artist to use it or using it makes you an artist. But in the sense there isn't an exact science to using it or mastering it.

I have good results with mine because I tinker. I tried to AFMA it by the book and I could NOT get a sharp shot out of it.90% of the time I use an AF point that would line up somewhere inline with the outer third of the frame so I AFMA with one of those points. I do that on a tripod, focusing on a ruler. Then I do some test shots handheld and tinker with the AFMA little by little until I get sharp shots consistently. This may also help compensate for repetitive flaws in my technique.I find shooting in servo can also help get sharp shots, even if you subjects are stationary.

I've been eyeballing getting the Canon 50mm L lens, the f/1.2 one.Reading the reviews on Amazon.com...I seen references about the lens having a problem with backfocusing.

Can someone explain this to me...what it is, why it happens?

Have others experienced this with this lens? Is it a bug through all of these lenses, or does it happen here and there?

Is this a reason to avoid this lens? If so..why hasn't Canon fixed this?

Thank you in advance,

cayenne

neuroanatomist below - as always - has given you a great technical explanation already. Let me ad my personal experience: I always loved 50mm lenses already back in the film days. The 50 1.4 has been a reference lens for decades and rightly so. In the EF/EOS world, unfortunately, this great lens has been reduced to a consumer grade plastic lens with a stupid AF clutch design that needs to treated like a raw egg. That was the main reason for me to upgrade to the 50L. And by upgrade I first and foremost mean the build quality which to me personal is very important even though I treat this one like a raw egg as well. Go figure.It is also an upgrade optically for my specific interests. It is much sharper wide open and the contrast is very special. It's a different lens and a different design. All this comes at a price (and I don't mean just the extra cost). My 1.4 in fact was sharper when shooting in the 2.0 or 2.8 range. Not just for brick-wall test shots but also in real life. So I kept it around for a while. I never used it while the 50L rarely is taken off my 5DII. So I sold the 1.4 even though it was a really good copy without any issues.

So despite this little difference when comparing it A to B the 50L is still the clear winner for me and I wouldn't want it changed in any way. It'll probably happen one day. Canon will come up with some (even more) plasticky new 1.4 design with IS and super sharp and everything. Doesn't interest me. I shoot wide open a lot and even stopped down to 1.6, 1.8 or 2.2 or whatever it still delivers stunning results. I've never noticed the back focus issue even though I now that by design it must be there. I've never even played around with the AF compensation settings. It's fine as it is. Just because of the narrow DOF there will be misses. So what? There will always be stunning hits also.

All the noise on various forums about how the 50L is junk because of the terrible back focus and the CA "issues", all the stupid comparisons of different manufacturing years, all the talk how the 5dII is horrid because of the AF and "banding". It's mostly irrelevant in real life. There is always something and you usually can make it work if you know what you want.

Make sure you know why you want this and then go try one out first. It's too expensive to just have it for the sake of having another hunk of glass with a red ring around it. If sharpness is your prime concern and you stop down usually a bit more the 50 1.4 may be a more reasonable choice. Or other manufacturers for that matter. Or other focal lengths such as the 35L or the 135L. "Sharper" but very different use.

jackreacher

All the noise on various forums about how the 50L is junk because of the terrible back focus and the CA "issues", all the stupid comparisons of different manufacturing years, all the talk how the 5dII is horrid because of the AF and "banding". It's mostly irrelevant in real life. There is always something and you usually can make it work if you know what you want.

Make sure you know why you want this and then go try one out first. It's too expensive to just have it for the sake of having another hunk of glass with a red ring around it. If sharpness is your prime concern and you stop down usually a bit more the 50 1.4 may be a more reasonable choice. Or other manufacturers for that matter. Or other focal lengths such as the 35L or the 135L. "Sharper" but very different use.

I couldn't agree more, well said! I love my 50L despite all the so-called issues

Wide open and focussed, all the rays passing through all parts of the lens focus (I think this is a pretty sharp lens?) to a sharp point on the sensor plane. Occlude some of the rays by stopping down the iris and the unoccluded rays seem to have their paths changed so they focus off the sensor plane.

Don't see how that could happen - maybe there's something different going on. Can soemone educate me on that?

Wide open and focussed, all the rays passing through all parts of the lens focus (I think this is a pretty sharp lens?) to a sharp point on the sensor plane. Occlude some of the rays by stopping down the iris and the unoccluded rays seem to have their paths changed so they focus off the sensor plane.

Don't see how that could happen - maybe there's something different going on. Can soemone educate me on that?

thank you Neuro and KBmelb for your answers. I have had this as one of my most wanted lenses for a long time. I will still get it after what you say, but not with eyes wide shut

I was in the same situation, contemplating getting the lens. In the end I bought it, and absolutely LOVE it! Honestly I don't even notice a back focus issue at all when stopping down, but then again I bought the lens to shoot at f1.2, if I wanted to stop down I'd buy a 1.4.

Zlatko

Focus shift is when the focal plane of the lens changes when you change the aperture of the lens. All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken. In the case of the 50mm f/1.2L, if you select an aperture narrower than f/1.2, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane. At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF. At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).

You are correct, but I've been using the 50/1.2L almost since it was introduced and have almost forgotten about the focus shift. It's just not much of an issue. I remember testing for focus shift when I first got the lens, and confirmed that it occurred at about f/2.8 at closer distances. I often shoot at f/2.2 to f/3.5, where it should be a problem, but it isn't because my subjects are usually not that close.

I could see it being more of an issue for some photographers. I believe Canon could fix this with a floating element. Perhaps an upgrade that we'll see in version II.

Wide open and focussed, all the rays passing through all parts of the lens focus (I think this is a pretty sharp lens?) to a sharp point on the sensor plane. Occlude some of the rays by stopping down the iris and the unoccluded rays seem to have their paths changed so they focus off the sensor plane.

Don't see how that could happen - maybe there's something different going on. Can soemone educate me on that?

It occurrs on all lenses, but the construction of the 50mmL and the wide aperture makes it a little more apparent. You cam mitigate it once you know whats happening and why."Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.Quite literally, the optimal plane of focus moves, depending on aperture! With every lens I’ve tested to date, the focus moves farther away. For example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it might now be centered at 1.02 meters. That apparently small difference means sparkling-sharp eyes versus not-quite-there eyes—it matters, especially with high-resolution digital cameras.Focus shift is caused by spherical aberration (see vanwalree.com for an excellent technical discussion). Instead of a sharply-focused point of light a spherically aberrated lens produces a point of light with a “halo”. This is visually confusing when focusing by eye (because of the lowered contrast) making it difficult to find optimal focus. It also is confusing to autofocus systems. In spite of these issues, accurate focus can generally be obtained—but it’s no longer accurate when the lens is stopped down."http://diglloyd.com/articles/Focus/FocusShift.html

It only happens when you stop down, but why pay for f1.2 and stop it down? UNLESS you're doing the ODD thing that needs some increased DOF, but at that point your probably at f8. The 50L is awesome, as Neuro said, its bokeh is smooooth.